Foamed porous membranes are usable for the filtration of fluids in the micro- and macrofiltration range, in particular for prefiltration and final filtration of liquid media in industry, in the laboratory, and in the environmental protection sector.
As described in WO 97/06935, predominantly closed-cell polymer foams with cell sizes of less than 0.1 xcexcm and cell densities of at least 1.56xc3x971014 cells/cm3 (supermicrocellular polymer foams), and with cell sizes of less than 100 xcexcm and cell densities of more than 108 cells/cm3 (microcellular polymer foams), can be manufactured by extrusion. The method is characterized by the steps of (a) forming a single-phase polymer/gas solution under pressure, (b) forming the nuclei for gas cells (cell nucleation) by pressure reduction, and (c) effecting cell growth. It is said to be essential to use a gear pump as a throttle valve between the extruder outlet and the nozzle, by which pressure reduction is controlled.
According to WO 92/17533, closed cell foamed films are produced from a foamable polymer and a supercritical fluid. Cell sizes of less than 1 xcexcm and cell densities between 109 and 1015 cells/cm are stated to be achieved. According to this very complex method, the film is produced by way of an extruder with a slit nozzle, a supercritical fluid such as CO2 is introduced at room temperature to a pressure chamber through which the film is guided via rollers, nucleation takes place upon passage into a second chamber at standard pressure, and cell growth is achieved in the second chamber at a temperature  greater than 190xc2x0 C. by feeding the film between heat exchangers and optionally by tempering. Alternatively, the supercritical fluid can also be introduced directly into the polymer melt inside the extruder, prior to extrusion by the nozzle.
WO 89/00918 describes the manufacture of closed-cell microcellular foams from semicrystalline polymers such as polypropylene and polyethylene with cell sizes varying from 1 to 100 xcexcm. According to the method, the polymer, at elevated pressure and above its melting point, is saturated with gas, the polymer material is shaped by way of an extrusion nozzle or by injection-molding, the pressure is reduced for cell nucleation and foaming, and the temperature is then reduced to below the polymer""s melting point in order to solidify the polymer foam. U.S. Pat. No. 4,473,665 discloses substantially the same method for the manufacture of closed-cell microcellular foams from amorphous polymers such as polystyrene, polyester, nylon, and polycarbonate, with cell sizes of approximately 2 to 25 xcexcm and a pore volume of 5 to 30%.
Such closed-cell polymer foams are, however, unsuitable for filtration membranes since, due to their closed-cell nature they exhibit virtually no, or at best a very low, flow rate or flux for a medium to be filtered, and possess insufficient pore volume. In addition, defects due to uncontrolled cell growth cannot be ruled out. A further disadvantage is that the closed-cells of such foams would need to be converted, in a further process, into an open-celled material in order to obtain usable membrane materials.
EP 0 754 488 A1 describes a method for producing open-celled microfiltration membranes from such closed-cell polymer foams, but the method carries with it the risk of membrane rupture. According to this method, the webs of material between the cells of the closed-cell polymer foams are broken by compressing and stretching the polymer foam at various temperatures. The flat membranes of polypropylene described therein, having a thickness of between 2 and 200 xcexcm, are said to have a wall thickness (B) to cell size (A) ratio of the polymer foam cells of less than 0.5, with a pore content not less than 50%.
WO 96/38221 discloses a method of making foamed polymer hollow fiber membranes. According to the method, a molten polymer is passed through an extrusion device, and the melt is charged with gas under pressure before entering a spinning nozzle which shapes the melt. The pressure drop upon emergence from the nozzle causes the polymer melt to foam, creating a porous hollow fiber membrane. The wall thickness of the hollow fiber is adjusted by stretching the hollow fiber membrane at an elevated temperature. The temperature of the melt in the extrusion device is regulated in such a way that for amorphous polymers it lies above the polymer""s glass transition temperature, and for partially crystalline polymers it lies above the polymer""s melting point. The size and shape of the pores are said to be adjustable by the extrusion parameters, such as pressure, temperature, extrusion screw shape and speed, as well as by the shape of the extrusion spinning nozzle. According to this method, foam structures having a pore size in the range from 10 to 20 xcexcm and cell densities of approximately 1010 cells/cm3, can be achieved. These are predominantly closed-cell foams with a randomly scattered proportion of open cells of between 5 and 40%. The disadvantages of this method are that the hollow-fiber membranes produced have inadequate pore volume and flux for technical applications, and defects due to uncontrolled cell growth are possible.
Accordingly, the objects of the present invention are to create defect-free foamed membranes made of thermoplastic polymers that have a large void fraction and a high proportion of open cells with a narrow pore size distribution, to provide a continuous method for manufacturing such membranes as well as an apparatus for carrying out the method. These objects and others which will become apparent to one of ordinary skill are summarized and described in detail below.
According to the present invention a polymer melt is formed comprising at least one amorphous and/or partially crystalline polymer, then delivered under pressure through an extrusion device, charged with a cell forming mixture and forced through a die to form a porous membrane. In response to the pressure drop occurring upon passage of the polymer melt through the die, the cell former foams the polymer melt, forming a predominantly open cellular structure in the polymer.